"You want a fair referee, or a strong referee anyway - and we didn't get that," Ferguson said on United's in-house TV channel. "I must say, when I saw who the referee was I feared it. I feared the worst."

Ferguson has until Tuesday to respond to a charge of "improper conduct relating to media comments."

If Ferguson is found guilty, it would trigger an additional two-match ban. Half of a four-match penalty for questioning referee Alan Wiley's fitness last season was suspended until the end of this season.